Chain extenders and formulations thereof for improving elongation in photosensitive polyimide

ABSTRACT

Photosensitive polymer formulations, materials and uses of such materials are disclosed. Embodiments of the present disclosure provide photosensitive polyimide materials having chain extenders and formulations thereof that improve elongation and formability of the polyimide materials, and methods of making such polymer materials.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of, and priority to, U.S.provisional patent application Ser. No. 62/868,761 entitled “ChainExtenders And Formulations Thereof for Improving Elongation inPhotosensitive Polyimide” filed on Jun. 28, 2019, the entire disclosureof which is hereby incorporated by reference.

TECHNICAL FIELD

Embodiments of the present disclosure relate generally to photosensitivepolyimide formulations, materials and uses thereof. More specifically,embodiments of the present disclosure relate to photosensitive polyimidematerials having chain extenders and formulations thereof that improveelongation and formability of the polyimide materials, and methods ofmaking.

BACKGROUND

Electronic circuits, such as printed circuit boards and the like areused in a wide range of components, and typically include conductive andinsulating layers. For example, in the disk drive industry, flexures arestructures that flexibly support a read/write transducer proximate arotating disk, while also supporting flexible electrical circuitry forconducting electrical signals to and from the transducer. In somestructures, a layer of stainless steel is included, sometimes as a baselayer, upon which various insulating and conductive layers are formed.

Polymer materials are widely used as insulating layers coated onto metallayers in the manufacture of electronic circuits. To create certainelectronic components, the conductive and insulating layers arepatterned using photolithography and etching techniques, and thusphotosensitive polymer materials are required. Polyimide materials havefound use as photosensitive polymer materials in the manufacture ofelectronic components. Suitable polyimide materials must satisfy manyparameters and properties, both during the initial manufacturingprocesses and later during subsequent processing and use.

In some instances, manufacture of electronic components requiresmechanical forming after the devices have been initially processed, andin such instances a cured polymer layer coated on the metal parts issubject to forming. Forming can include bending of the polymer coatedmetal parts up to an angle of approximately 90 degrees. A small formingradius is particularly desirable as device footprint decreases.

As the device is subjected to forming (e.g., bending of the device), thepolymer coating is elongated in the formed area. Forming of the polymercoating poses many challenges. A component or device is generallyincludes a polymer layer coated on a metal layer, such as stainlesssteel. When initially manufactured, the metal layer in the device is ina neutral axis state, in which the metal layer is neither in tension norcompression. At this stage, the polymer layer will have been cured.Next, the device is formed or bent which causes the polymer layer toelongate in the formed area. As the radius of the formed area continuesto decrease, the polymer elongation continues to increase in the formedarea.

If the polymer ultimate strain is exceeded during the forming process,cracks in the polymer layer will occur and the polymer material willfail. This is a significant problem and has severely limited the utilityof photosensitive polymer materials in such formed device applications.Common failure modes of polymer layers after forming include completebreaks in the polymer layer and partial breaks in the polymer layers,both in polymer layers formed atop a metal layer and in freestandingpolymer layers, respectively. Such complete and partial breaks in theformed polymer layers are catastrophic and can render the devicesunusable.

This problem is prevalent across conventional polymer materials. Verylow molecular weight polymer formulations tended to crack badly duringsolvent develop steps. On the other hand, it is known that highermolecular weight polymers exhibit better elongation and formabilityafter forming. However, when photosensitive materials are needed, highmolecular weight photosensitive polymers are difficult or impractical toprocess. By way of example and without limitation, high molecular weightpolymers are generally considered to have an average molecular weight inthe range of about 40,000 and above. High molecular weight polymersexhibit properties that negatively impact the initial manufacturingsteps, and in particular the photolithography processing steps. Forexample, high molecular weight photosensitive polymers generally exhibitvery high viscosity and/or low solids content. Filtration is very timeconsuming and expensive. Photolithographic contrast and develop speedare poor. Thus, high molecular weight photosensitive polymers are notsuitable for the initial processing steps prior to the cure step.

Thus, the problem is highly complicated and presently available polymermaterials are substantially unsuitable. According, new developments aregreatly needed.

SUMMARY

Broadly, embodiments of the present disclosure provide photosensitivepolymer formulations, materials and uses of such materials. Morespecifically, embodiments of the present disclosure providephotosensitive polyimide materials having latent chain extenders andformulations thereof that improve elongation and formability of thepolyimide materials, and methods of making such polymer materials.

The inventors have discovered that a number of complex factors must beunderstood and balanced with respect to the chemistry and properties ofphotosensitive polymer formulations throughout processing, includingduring the initial device manufacturing steps, the manner in which thepolymer formulation undergoes the curing step, and the subsequentforming step, in order to solve the aforementioned problems.

As described in more detail below, polymers having lower molecularweights exhibit more desirable characteristics during initialmanufacturing steps, in particular photolithography processing steps.While polymers having higher molecular weights exhibit better elongationor formability during subsequent forming step. After substantial studyand effort, the inventors have developed an innovative polymerformulation and method of making, wherein the polymer formulation andchemistry is selectively manipulated to control the molecular weight ofthe polymer to achieve desired properties throughout the manufacturing,curing, and forming steps. For example, in some embodiments a method isprovided which selectively controls the molecular weight of a poly(amicacid) in the photosensitive polymer formulation to be in a relativelylow weight average molecular weight range during the initial processingsteps, and then increases the weight average molecular weight of thepolyimide polymer during curing to form a polyimide insulating layerthat contains a polyimide polymer having a high weight average molecularweight that exhibits improved elongation and formability during asubsequent forming step. Thus, broadly stated the molecular weight ofthe polyimide polymer is greater (increased) after curing.

In some embodiments, the weight average molecular weight of thepolyimide polymer is increased by at least two times after curing ascompared to the weight average molecule weight of the base polymer inthe initial polymer formulation before curing. In some embodiments, theweight average molecular weight of the polyimide polymer is increased toabout 60,000 after curing. For some embodiments, the weight averagemolecular weight of the polyimide polymer can be increased to much morethan 60,000, e.g., 10,000,000, after curing. While the weight averagemolecular weight of the cured polymer cannot be readily measured, it canbe estimated based on comparison to a known polymer with similar formingbehavior.

For some embodiments, a photosensitive polymer formulation can include apoly(amic acid) salt as a polyimide precursor, and a tertiary amine saltof a tetracarboxylic acid as a latent chain extender.

For some embodiments, the poly(amic acid) salt includes (i) a basepolymer of a dianhydride and a diamine, and (ii) a crosslinking agent.

For some embodiments, the poly(amic acid) salt is a poly(amic acid)tertiary amine salt.

For some embodiments, the dianhydride is BPDA.

For some embodiments, the diamine is TFMB.

For some embodiments, the crosslinking agent is DEEM.

For some embodiments, a weight average molecular weight of the basepolymer is about 40,000 and less.

For some embodiments, a weight average molecular weight of the basepolymer is about 25,000-35,000.

For some embodiments, the tertiary amine salt of a tetracarboxylic acidas the latent chain extender is prepared by reacting a dianhydride withwater and a tertiary amine at room temperature. For some embodiments,the tertiary amine salt of a tetracarboxylic acid as the latent chainextender is prepared by reacting a tetracarboxylic acid with a tertiaryamine.

For some embodiments, the latent chain extender is prepared by reactingBPDA with water and DEEM at room temperature to form the tertiary aminesalt of a tetracarboxylic acid.

For some embodiments, a mol % of the diamine is higher than the mol % ofthe dianhydride.

For some embodiments, a mol % of the dianhydride to a mol % of thediamine is in the range of 0.900 to 0.999:1.000.

For some embodiments, a ratio of a mol % of the dianhydride and thelatent chain extender to a mol % of the diamine is about 1.000:1.000.

For some embodiments, the photosensitive polymer formulation can furtherinclude a photoinitiator.

For some embodiments, the photosensitive polymer formulation can furtherinclude a sensitizer.

For some embodiments, the photosensitive polymer formulation can furtherinclude a dissolution accelerator.

For some embodiments, the photosensitive polymer formulation can furtherinclude a adhesion promoter.

For some embodiments, a method of forming a polyimide polymer isprovided that can include: providing the photosensitive polymerformulation of according to some embodiments of the present disclosure;and curing the photosensitive polymer formulation, whereupon the latentchain extender is activated and reforms a dianhydride that reacts withterminal amine groups to form new imide bonds forming the polyimidepolymer.

For some embodiments, the poly(amic acid) salt includes (i) a basepolymer of a dianhydride and a diamine, and (ii) a crosslinking agent.

For some embodiments, the method can further include controlling a mol %of the diamine to be higher than a mol % of the dianhydride to control aweight average molecular weight of the base polymer of the poly(amicacid) salt in the photosensitive polymer formulation.

For some embodiments, the method can further include controlling a ratioof a mol % of the dianhydride and the latent chain extender to a mol %of the diamine is about 1.000:1.000.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, exemplify various embodiments of the presentinvention and, together with the description, serve to explain andillustrate principles of the invention. The drawings are intended toillustrate major features of the exemplary embodiments in a diagrammaticmanner. The drawings are not intended to depict every feature of actualembodiments nor relative dimensions of the depicted elements, and arenot generally drawn to scale.

FIG. 1 shows an exemplary reaction of forming the latent chain extenderaccording to some embodiments of the present disclosure;

FIG. 2 shows an exemplary reaction for forming a polymer with higherweight average molecular weight from the polyimide precursor and thelatent chain extender according to some embodiments of the presentdisclosure.

DETAILED DESCRIPTION

To solve the problems described in the summary, the inventors have spentsubstantial effort investigating parameters, characteristics andvariables that impact various properties of polymer materials duringdifferent manufacturing process steps. For purposes of this description,the manufacturing process steps are broadly defined as: (1) initialdevice manufacturing steps which include (without limitation) coating aninitial polymer formulation on metal layers, and variousphotolithography steps such as patterned UV exposure and developing; (2)curing of the polymer formulation on the metal layer; and (3) subsequentforming step. Those of skill in the art will recognize that other stepsmay be employed and in varying order and that the inventions disclosedherein are not limited to such, but that the above definition isprovided for convenience and ease of description.

A polymer formulation including a polyimide precursor and a latent chainextender is provided. For some embodiments, the polyimide precursor is apoly(amic acid) salt. Preferably, the polyimide precursor is a poly(amicacid) salt including (i) a polymer of a dianhydride and a diamine, and(ii) a crosslinking agent. For some embodiments, the polyimide precursoris a poly(amic acid) tertiary amine salt. The polyimide precursor can bemade from synthesis methods known in the art. For example, the polyimideprecursor can be made from a synthesis method in which the dianhydrideand diamine are polymerized in an organic solvent to produce a poly(amicacid) solution. Then, a crosslinking agent, such as a tertiary amine, isadded to form a poly(amic acid) tertiary amine salt solution, as well asany other additives that are part of the photopackage.

The poly(amic acid), i.e., the polymer of a dianhydride and a diamine,has relatively low weight average molecular weight. By way of exampleand without limitation, low weight average molecular weight polymersaccording to the present disclosure are generally considered to have aweight average molecular weight in the range of about 40,000 and less,typically about 25,000-35,000. It should be noted that for the weightaverage molecular weight numbers disclosed in the present disclosure,the numbers were determined using gel permeation chromatography (GPC)using DMF with LiBr and phosphoric acid as the mobile phase and based onPEO standards. It should also be understood within the scope of thedisclosed invention, that for high weight average molecular weightpolymers, the molecular weight needed for elongation performance willchange with the polymer backbone.

The dianhydride of the polyimide precursor includes may include, but isnot limited to, monomers having an anhydride structure. Preferably, thedianhydride includes a tetracarboxylic acid dianhydride structure. Thedianhydride component employed may be any suitable dianhydride forforming a crosslinkable or crosslinked polyimide prepolymer, polymer, orcopolymer. For example, tetracarboxylic acid dianhydrides, singly or incombination, may be utilized, as desired.

For some embodiments, the dianhydride is an aromatic dianhydride.Illustrative examples of aromatic dianhydrides suitable for use in thepolyimide precursor include:2,2-bis(4-(3,4-dicarboxyphenoxy)phenyl)propane dianhydride;4,4′-bis(3,4-dicarboxyphenoxy)diphenyl ether dianhydride;4,4′-bis(3,4-dicarboxyphenoxy)diphenyl sulfide dianhydride;4,4′-bis(3,4-dicarboxyphenoxy)benzophenone dianhydride;4,4′-bis(3,4-dicarboxyphenoxy)diphenyl sulfone dianhydride;2,2-bis(4-(2,3-dicarboxyphenoxy)phenyl)propane dianhydride;4,4′-bis(2,3-dicarboxyphenoxy)diphenyl ether dianhydride;4,4′-bis(2,3-dicarboxyphenoxy)diphenyl sulfide dianhydride;4,4′-bis(2,3-dicarboxyphenoxy)benzophenone dianhydride;4,4′-bis(2,3-dicarboxyphenoxy)diphenyl sulfone dianhydride;4-(2,3-dicarboxyphenoxy)-4′-(3,4-dicarboxyphenoxy)diphenyl-2,2-propanedianhydride; 4-(2,3-dicarboxyphenoxy)-4′-(3,4-dicarboxyphenoxy)diphenylether dianhydride;4-(2,3-dicarboxyphenoxy)-4′-(3,4-dicarboxyphenoxy)diphenyl sulfidedianhydride;4-(2,3-dicarboxyphenoxy)-4′-(3,4-dicarboxyphenoxy)benzophenonedianhydride and4-(2,3-dicarboxyphenoxy)-4′-(3,4-dicarboxyphenoxy)diphenyl sulfonedianhydride, 1,2,4,5-benzenetatracarboxylic dianhydride as well asmixtures comprising one of the foregoing dianhydrides.

Preferred dianhydrides include the following dianhydride compounds:

-   -   3,4,3′,4′-biphenyltetracarboxylic dianhydrides (BPDA) having the        following formula:

-   -   3,4,3′,4′-benzophenonetetracarboxylic dianhydrides (BTDA) having        the following formula:

-   -   2,2-bis(3′,4′-dicarboxyphenyl)hexafluoropropane dianhydrides        (6FDA) having the following formula.

-   -   pyromellitic dianhydrides (PMDA) having the following formula:

-   -   and mixtures thereof.

The diamine is a diamine compound having two amino groups in themolecular structure. Examples of the diamine compound include anyaromatic diamine compound or aliphatic diamine compound, with anaromatic diamine compound being preferable.

Examples of the diamine compound include aromatic diamines such as2,2′-bis(trifluoromethyl)benzidine (TFMB), p-phenylenediamine,m-phenylenediamine, 4,4′-diaminodiphenylmethane,4,4′-diaminodiphenylethane, 4,4′-diaminodiphenyl ether,4,4′-diaminodiphenylsulfide, 4,4-diaminodiphenylsulfone,1,5-diaminonaphthalene, 3,3-dimethyl-4,4′-diaminobiphenyl,5-amino-1-(4′-aminophenyl)-1,3,3-trimethylindane,6-amino-1-(4′-aminophenyl)-1,3,3-trimethylindane,4,4′-diaminobenzanilide, 3,5-diamino-3′-trifluoromethylbenzanilide,3,5-diamino-4′-trifluoromethylbenzanilide, 3,4′-diaminodiphenyl ether,2,7-diaminofluorene, 2,2-bis(4-aminophenyl)hexafluoropropane,4,4′-methylene-bis(2-chloroaniline),2,2′,5,5′-tetrachloro-4,4′-diaminobiphenyl,2,2′-dichloro-4,4′-diamino-5,5′-dimethoxybiphenyl,3,3′-dimethoxy-4,4′-diaminobiphenyl, 4,4′-diamino2,2′-bis(trifluoromethyl)biphenyl,2,2-bis[4-(4-aminophenoxy)phenyl]propane,2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane,1,4-bis(4-aminophenoxy)benzene, 4,4′-bis(4-aminophenoxy)-biphenyl,1,3′-bis(4-aminophenoxy)benzene, 9,9-bis(4-aminophenyl)fluorene,4,4′-(p-phenyleneisopropylidene)bisaniline,4,4′-(m-phenyleneisopropylidene)bisaniline,2,2′-bis[4-(4-amino-2-trifluoromethylphenoxy)phenyl]hexafluoropropane,and 4,4′-bis[4-(4-amino-2-trifluoromethyl)phenoxy]-octafluorobiphenyl;aromatic diamines having two amino groups bonded to an aromatic ring andhetero atoms other than nitrogen atoms of the amino groups such asdiaminotetraphenyl thiophene; and aliphatic and alicyclic diamines suchas 1,1-metaxylylenediamine, 1,3-propanediamine, tetramethylenediamine,pentamethylenediamine, octamethylenediamine, nonamethylenediamine,4,4-diaminoheptamethylenediamine, 1,4-diaminocyclohexane,isophoronediamine, tetrahydrodicyclopentadienylenediamine,hexahydro-4,7-methanoindanylene dimethylenediamine,tricyclo[6,2,1,0^(2,7)]-undecylene dimethyldiamine, and4,4′-methylenebis(cyclohexylamine).

Among these, an aromatic diamine compound is preferable as the diaminecompound. Specifically, for example, 2′-bis(trifluoromethyl)benzidine(TFMB), p-phenylenediamine, m-pheylenediamine,4,4′-diaminodiphenylmethane, 4,4′-diaminodiphenyl ether,3,4′-diaminodiphenyl ether, 4,4′-diaminodiphenylsulfide, and4,4′-diaminodiphenylsulfone are preferable, and 4,4′-diaminodiphenylether and p-phenylenediamine are particularly preferable. Mostpreferably, the diamine compound is TFMB. The diamine compound may beused alone or in combination of two or more kinds thereof.

The crosslinking agent forming the poly(amic acid) salt can be anysuitable crosslinking agent known in the art. For some embodiments, thecrosslinking agent forming the poly(amic acid) salt is a tertiary amine,thereby forming poly(amic acid) tertiary amine salt as the polyimideprecursor. Suitable crosslinking agents for the polymer formulationinclude, but are not limited to, 2-(diethylamino)ethyl methacrylate(DEEM), 2-(dimethylamino)ethyl methacrylate (DMAEMA),2-(dimethylamino)ethyl methacrylate (DMEM), 3-(dimethylamino)propylmethacrylate (DMPM), 2-(dimethylamino)ethyl acrylate (DMEA),2-(diethylamino)ethyl acrylate (DEEA), and 3-(dimethylamino)propylacrylate (DMPA). Preferably, the crosslinking agent is2-(diethylamino)ethyl methacrylate (DEEM).

These exemplary tertiary amines include a double bond, which can serveas a crosslinking agent. Other tertiary amines which include doublebonds can also be used and can also serve as a crosslinking agent. Thetertiary amine can also be an additive or part of an additive thatinfluences other properties of the final resulting polyimide.

The polyimide precursor can be provided in a solvent as discussed above.Any suitable solvent that is known in the art can be used. For someembodiments, the solvent can be an organic solvent, including, but notlimited to, N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF),N,N-dimethylacetamide (DMAc), dimethylsulfoxide (DMSO),hexanemethylenephosphoramide (HMPA), N-methylcaprolactam, andN-acetyl-2-pyrrolidone.

The polymer formulation further includes a latent chain extender alongwith the polyimide precursor. For some embodiments, the latent chainextender is a tertiary amine salt of a tetracarboxylic acid. For someembodiments, the latent chain extender can be prepared by reacting adianhydride of choice with water and a tertiary amine cross-linker atroom temperature. The dianhydride used to form the latent chain extendercan be a dianhydride as discussed above. For purposes of the presentdisclosure, room temperature is about 20 to 24° C.

As an illustrative embodiment of the latent chain extender, adianhydride of choice, such as BPDA, is reacted with water and atertiary amine cross-linker at room temperature to produce a tertiaryamine salt of a tetracarboxylic acid, as shown in the reaction providedin FIG. 1. More specifically, in the exemplary reaction provided in FIG.1, the tertiary amine salt of a tetracarboxylic acid (chain extender) isprepared by reacting BPDA with water and 2-(diethylamino)ethylmethacrylate (DEEM) in a solvent, such as NMP.

The tertiary amine can be a cross-linker, such as 2-(diethylamino)ethylmethacrylate (DEEM) or 2-(dimethylamino)ethyl methacrylate (DMAEMA).These exemplary tertiary amines include a double bond, which can serveas a cross-linker. Other tertiary amines which include double bonds canalso be used and can also serve as a cross-linker. The tertiary aminecan also be an additive or part of an additive that influences otherproperties of the final resulting polyimide.

Any suitable solvent that is known in the art can be used. For someembodiments, the solvent can be an organic solvent, including, but notlimited to, N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF),N,N-dimethylacetamide (DMAc), dimethylsulfoxide (DMSO),hexanemethylenephosphoramide (HMPA), N-methylcaprolactam, andN-acetyl-2-pyrrolidone.

The chain extender is selected such that it is latent at roomtemperature, meaning that no reaction occurs between the polyimideprecursor and the chain extender unless heated to a curing temperature(e.g., 250 to 450° C.). This polymer formulation having the latent chainextender is then used during the initial device manufacturing(photolithography) steps where a low weight molecular weight polymer isdesired. Once the photolithography steps are complete, the polymerformulation is cured. Upon curing of the polymer formulation, the chainextender becomes activated, meaning the dianhydride is reformed as shownin the exemplary reaction of FIG. 1. The reformed dianhydride reactswith terminal amine groups to form new imide bonds, thereby forming ahigher weight average molecular weight polymer. For exemplary purposes,FIG. 2 provides an exemplary reaction for forming the polymer withhigher weight average molecular weight from the polyimide precursor andthe latent chain extender. The exemplary polyimide precursor isBPDA-TFMB (amic acid) salt with 2-(diethylamino)ethyl methacrylate(DEEM).

In the polyimide precursor, the mol % of the diamine compound ispreferably higher than the mol % of the dianhydride. Regarding the ratioof the mol % of the dianhydride to the diamine compound, the mol % ofthe dianhydride to the mol % of the diamine compound is preferably inthe range of 0.900 to 0.999:1.000, and more preferably in the range of0.950 to 0.990:1.000.

In the polyimide precursor, the molar equivalent of the diamine compoundand the molar equivalent of the dianhydride are measured by techniquesknown in the art. For example, the polyimide precursor resin can besubjected to a hydrolysis treatment in a basic aqueous solution ofsodium hydroxide and potassium hydroxide in order to be decomposed intodiamine compound and dianhydride. The obtained sample is analyzed by gaschromatography, liquid chromatography, or the like, and the proportionsof the dianhydride and the diamine compound constituting the polyimideprecursor are determined.

For some embodiments, the polymer formulation includes the latent chainextender (forming a dianhydride at a curing temperature) in a mol % suchthat the ratio of the mol % of the dianhydride and the latent chainextender to the mol % of the diamine compound is about 1.000:1.000.

To determine the mol % of the latent chain extender (forming adianhydride at a curing temperature) can be predicted by the Carothersequation. That is, the diamine and the dianhydride monomer are generallyadded in approximately a 1:1 molar stoichiometry to maximize averagepolymer chain length. Adding the diamine and dianhydride monomer inother ratios can change the average chain length of the resultingpolymer. In general, polymerization reactions conducted withstoichiometries outside of essentially a 1:1 molar ratio of the monomersreduces the average polymer chain length as predicted by the Carothersequation and verified extensively in practice.

The Carothers equation is provided below

$M_{n} = {{M_{o}X_{n}\mspace{14mu} X_{n}} = \frac{1 + r}{1 - r}}$

-   -   M_(n)=number average molecular weight    -   M₀=molecular weight of the repeat unit    -   X_(n)=number average degree of polymerization    -   r=stoichiometric ratio of monomers (r≤1)

According to some embodiments of the present disclosure, in thepolyimide precursor, the mol % of the diamine compound is preferablyhigher than the mol % of the dianhydride as discussed above. Through theCarothers equation, the mol % of the latent chain extender (forming adianhydride at a curing temperature) can be predicted such that thediamine and the dianhydride monomer are generally added in approximatelya 1:1 molar stoichiometry. In other words, the ratio of the mol % of thedianhydride and the latent chain extender to the mol % of the diaminecompound is about 1.000:1.000.

For some embodiments, the polymer formulation includes the latent chainextender in an amount from 0.1 mol % to 10.0 mol % such that the ratioof the mol % of the dianhydride and the latent chain extender to the mol% of the diamine compound is about 1.000:1.000. For some embodiments,the polymer formulation includes the latent chain extender in an amountfrom 0.5 mol % to 5.0 mol % or 1.0 mol % to 3.0 mol % such that theratio of the mol % of the dianhydride and the latent chain extender tothe mol % of the diamine compound is about 1.000:1.000.

For some embodiments, the crosslinking agent for forming the poly (amicacid) salt, i.e., polyimide precursor, can be contained in the polymerformulation from about 0.1 to 10.0 moles per mole dianhydride. For someembodiments, the crosslinking agent from about 0.5 to 7.5 moles, 1.0 to5.0 moles, or 1.0 to 3.0 moles per mole dianhydride.

For some embodiments, the polymer formulation optionally includes otheradditives, such as a photoinitiator, a sensitizer, a dissolutionaccelerator, and an adhesion promoter. The photoinitiator can be asuitable photoinitiator known in the art. For some embodiments, thephotoinitiator generates radicals upon UV exposure. Preferably, aphotoinitiator absorbance peak is located where a PI precursorabsorbance is minimal. For example, suitable photoinitiators for thepolymer formulation include, but not limited to,1-[9-ethyl-6-(2-methylbenzyl)-9H-carbazol-3-yl]ethanone-1-(O-acetyloxime)(OXE-02)and 1,2-octanedione 1-[4-(phenylthio)phenyl]2-(O-benzoyloxime) (OXE-01),various phosphine oxides, such asdiphenyl(2,4,6-trimethylbenzoyl)phosphine oxide andphenylbis(2,4,6-trimethylbenzoyl)phosphine oxide, various acetophenonebased initiators including2-benzyl-2-(dimethylamino)-4′-morpholinobutyrophenone,2,2-dimethoxy-2-phenylacetophenone, 2-hydroxy-2-methylpropiophenone, and2-methyl-4′-(methylthio)-2-morpholinopropiophenone, various benzoinbased initiators including benzoin, benzoin methyl ether, benzoin ethylether, 4,4′-dimethoxybenzoin, 4,4′-dimethylbenzil, benzophenone basedinitiators including benzophenone, 4,4′-bis(dimethylamino)benzophenone(Michler's ketone), 4,4′-bis(diethylamino)benzophenone (Michler's ethylketone), 4-(dimethylamino)benzophenone, 4-(diethylamino)benzophenone,4,4′-dihydroxybenzophenone, and various hydroxybenzophenones andalkylbenzophenones, thioxanthones including thioxathen-9-one, isopropylthioxanthen-9-ones, diethyl thioxanthen-9-ones and chlorothioxanthene-9-ones. Preferably, the photoinitiator is OXE-01, OXE-02,diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide,phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide,4,4′-bis(dimethylamino)benzophenone, and4,4′-bis(diethylamino)benzophenone. Most preferably, the photoinitiatoris OXE-02.

For some embodiments, the photoinitiator can be contained in the polymerformulation from about 0.1 to 10.0 parts by weight per 100 parts byweight of poly(amic acid). For some embodiments, the photoinitiator canbe contained in the polymer formulation in an amount from about 0.5 to7.5 or 1.0 to 5.0 parts by weight per 100 parts by weight of poly(amicacid).

The sensitizer can be any suitable sensitizer known in the art. For someembodiments, the sensitizer is a tertiary amine containing an aromaticor (meth)acrylate group to stabilize a radical ion formed in thephotoreaction. As there is already an abundance of a tertiary amine(meth)acrylate in the formulation, the addition of sensitizer in theformulation is likely not needed or optional. Suitable sensitizers forthe polymer formulation include, but not limited to, N-Phenyldiethanolamine (NPDEA), N-phenyl glycine, Michler's ketone, Michler'sethyl ketone, and various alkyl 4-(dimethylamino)benzoates. For someembodiments, the crosslinkers described above can also be consideredsensitizers for the polymer formulation.

For some embodiments, the sensitizer can be contained in the polymerformulation from about 0.1 to 10.0 parts by weight per 100 parts byweight of poly(amic acid). For some embodiments, the sensitizer can becontained in the polymer formulation in an amount from about 0.5 to 7.5or 1.0 to 5.0 parts by weight per 100 parts by weight of poly(amicacid).

The dissolution accelerator can be any suitable dissolution acceleratorknown in the art. For some embodiments, the dissolution accelerator is asmall molecule that has sufficiently low volatility to remain in a castfilm during a post apply bake. For example, suitable dissolutionaccelerators for the polymer formulation include, but not limited to,triethylene glycol dimethacrylate (TEGMA), benzotriazole (BTA), andtrimethylolpropane trimethacrylate (PTMA). Other examples include, butare not limited to, tetraethylene glycol dimethacrylate, triethyleneglycol triacrylate, tetraethylene glycol dimethacrylate, various tri-and tetra-(meth)acrylate esters of pentaeryritol, 2-nitrobenzaldehydeand 3-nitrobenzaldehyde, dihydropyridine derivatives, and aromaticsulfonamides.

For some embodiments, the dissolution accelerator can be contained inthe polymer formulation from about 0.1 to 20.0 parts by weight per 100parts by weight of poly(amic acid). For some embodiments, thedissolution accelerator can be contained in the polymer formulation inan amount from about 0.5 to 15.0 or 1.0 to 10.0 parts by weight per 100parts by weight of poly(amic acid).

The adhesion promoter can be any suitable adhesion promoter known in theart. For some embodiments, the adhesion promoter is alkoxysilane basedwith primary amine functionality. For some embodiments, the adhesionpromoter is a silicon-containing diamine as it will incorporatethroughout the PI backbone and will not interfere with the chainextension mechanism. Examples of monofunctional amine adhesion promotersinclude 4-aminobutyltriethoxysilane,4-amino-3,3′-dimethylbutyltrimethoxysilane,3-(m-aminophenoxy)propyltrimethoxysilane, m-aminophenyltrimethoxysilane,p-aminophenyltrimethoxysilane, 3-aminopropyltrimethoxysilane,11-aminoundecyltriethoxysilane, 3-aminopropylmethyldiethoxysilane.Exemplary silicon-containing diamines include1,3-bis(3-aminopropyl)tetramethyldisiloxane and its isomers(2-aminopropyl groups are a common impurity), as well as aminopropylterminated oligomers of polydimethylsiloxane. Most preferably, theadhesion promoter is isomerically pure1,3-bis(3-aminopropyl)tetramethydisiloxane due to thermal stability.

For some embodiments, the adhesion promoter can be contained in thepolymer formulation from about 0.1 to 5.0 parts by weight per 100 partsby weight of poly(amic acid). For some embodiments, the adhesionpromoter can be contained in the polymer formulation in an amount fromabout 0.1 to 3.0 or 0.1 to 1.0 parts by weight per 100 parts by weightof poly(amic acid).

A method for of controlling molecular weight of a photosensitivepolyimide formulation before and after curing is also provided. Themethod includes providing an initial polymer formulation as discussedabove. The polymer formulation includes the polyimide precursor and thelatent chain extender as discussed above where a low molecular weightpolymer is needed during the initial device manufacturing(photolithography) steps. The poly(amic acid), i.e., the polymer of adianhydride and a diamine, which is part of the polyimide precursor, hasrelatively low weight average molecular weight. By way of example andwithout limitation, low weight average molecular weight polymersaccording to the present disclosure are generally considered to have aweight average molecular weight in the range of about 40,000 and less,typically about 25,000-35,000.

To control the weight average molecular weight of the poly(amic acid) ofthe polyimide precursor, the mol % of the diamine compound is controlledsuch that it is higher than the mol % of the dianhydride in the initialpolymer formulation. Through the Carothers equation, the mol % of thelatent chain extender (forming a dianhydride at a curing temperature) iscontrolled such that the diamine and the dianhydride monomer aregenerally added in approximately a 1:1 molar stoichiometry. In otherwords, the ratio of the mol % of the dianhydride and the latent chainextender to the mol % of the diamine compound is about 1.000:1.000.

Once the photolithography steps are complete, the polymer formulation iscured. Upon curing of the polymer formulation, the chain extenderbecomes activated. The chain extender reforms a dianhydride that reactswith terminal amine groups to form imide bonds to form a highermolecular weight polymer than the poly(amic acid) of the polyimideprecursor.

In addition, a method for making a device using the polymer formulationdescribed above is provided. The method for making the device includes,coating, exposing, developing, curing, etching, and mechanical forming.For some embodiments, coating includes applying a polyimide coating on asubstrate. For some embodiments, the polyimide coating is the polymerformulation discussed in detail above. Preferably, the substrate is astainless steel substrate; however, other suitable substrates known inthe art are also appropriate. The polyimide coating can be applied usingtechniques including, but not limited to, liquid slot die, roller coat,spray, curtain coat, dry film lamination, and screen-printingtechniques. For some embodiments, the polyimide coating is applied byliquid slot die and then, exposed to UV light, developed with anappropriate solvent known in the art, and cured.

Curing of the polyimide coating, e.g., the polymer formulation discussedabove, to form a polyimide insulating layer on the substrate can beperformed by any suitable techniques known in the art, such as infraredcuring. As shown in the exemplary reaction provided in FIG. 2, duringthe cure step, DEEM and water are given off and the chain extenderreverts back to dianhydride. The reformed dianhydride reacts withterminal amine groups of the poly(amic acid) to form imide bonds, whichincreases the molecular weight of the polyimide polymer. Thus, theresultant polyimide exhibits relatively high molecular weight andimproved elongation and formability properties necessary for subsequentforming steps.

For some embodiments, the method for making the device further includesapplying a resist coating on the substrate. The resist coating isapplied on the base material using techniques including, but not limitedto, liquid slot die, roller coat, spray, curtain coat, dry filmlamination, and screen-printing techniques. The resist coating is thenexposed to UV light, developed, etched (that is, the substrate is etchedin regions not protected by the resist pattern), and stripped usingphotolithography and etching techniques including those known in theart. For some embodiments, the method includes mechanical forming of thedevices. According to some embodiments, parts were formed with a 100 μmradius to test the formability of the parts.

Examples

The following is an exemplary procedure for preparing the polymerformulation of the present disclosure. In a 300 mL reactor purged withnitrogen, NMP (100 mL) was added. TFMB (28.80 g) was measured out in abeaker then added to the reactor with vigorous stirring. Additional NMP(40 mL) was added to the beaker to dissolve any TFMB adhered to thebeaker, and this was then added to the reactor. After completedissolution of the TFMB, BPDA (25.20 g) was slowly added to the reactor,along with additional NMP (40 mL) to ensure quantitative transfer ofBPDA. The reactor temperature was set to 70° C. and the solution wasstirred for 4 hours.

In a separate flask under nitrogen, BPDA (1.27 g) was suspended in NMP(30 mL). DEEM (3.22 g) and water (0.156 g) were added and contents werestirred for 4 hours at room temperature until everything was dissolved.OXE-02 (1.62 g) was then added and stirred until dissolved to form thephotopackage solution.

After the 300 mL reactor had returned to room temperature, DEEM (31.90g) was added dropwise to the reactor over 30 minutes. Finally, thephotopackage solution was added to the reactor, and the solution wasstirred for 1 hour at room temperature before degassing under vacuum,bottling, and freezing.

The polymer formulations of Examples 1-3 having a composition as shownin Tables 1-2 is prepared by the exemplary procedure discussed above.Comparative Examples 1-13 are prepared in a similar manner, but do notinclude the latent chain extender in their composition. The compositionsof Comparative Examples 1-13 are also shown in Tables 1-2. All of theExamples and Comparative Examples have about 18 wt % poly(amic acid)solutions in NMP solvent and about 28-29 wt % poly(amic acid) DEEM saltsolutions after addition of the DEEM cross-linking agent.

TABLE 1 Dissolution Adhesion Latent Crosslinking PhotoinitiatorSensitizer accelerator Promoter Chain Agent (moles (parts per (parts per(parts per (parts per BPDA TFMH Extender per mole 100 parts 100 parts100 parts 100 parts (mol %) (mol %) (mol %) di

hydride PAA) PAA PAA) PAA) Example 1 47.6 50 2.4 2.0 3 0 0 0 Example 247.6 50 2.4 2.0 3 0 2 0 Example 3 47.6 50 2.4 2.0 3 0 2 0 Comparative 5050 0 2.0 3 0 5 0 example 1 Comparative 49.1 50.9 0 2.0 3 0 5 0 example 2Comparative 49.0 51.0 0 2.0 3 0 5 0 example 3 Comparative 48.8 51.2 02.0 3 0 10 0 example 4 Comparative 48.8 51.2 0 2.0 3 3 10 0 example 5Comparative 48.8 51.2 0 2.0 3 3 10 0.5 example 6 Comparative 48.8 51.2 02.0 3 0 10 0 example 7 Comparative 48.8 51.2 0 2.0 3 0 10 0 example 8Comparative 48.8 51.2 0 2.34 3 0 0 0 example 9 Comparative 48.8 51.2 02.0 3 0 5 0 example 10 Comparative 48.8 51.2 0 2.0 3 0 10 0 example 11Comparative 48.8 51.2 0 2.0 3 0 5 0 example 12 Comparative 48.8 51.2 02.34 3 0 0 0 example 13

indicates data missing or illegible when filed

TABLE 2 Latent % Parts Crosslinking Chain Dissolution Adhesion MW withSample Polymer Agent Extender Photoinitiator Sensitizer AcceleratorPromoter (g/mol) cracks Example 1 BPDA-TFMB DEEM BPDA- OXE02 N/A N/A N/A32.9K  0% Example 2 BPDA-TFMB DEEM BPDA- OXE02 N/A BTA N/A 26.6K  0%Example 3 BPDA-TFMB DEEM BPDA- OXE02 N/A BTA N/A 25.4K 10% ComparativeBPDA-TFMB DEEM N/A OXE02 N/A BTA N/A 59.8K  3% example 1 ComparativeBPDA-TFMB DEEM N/A OXE02 N/A BTA N/A 41.6K 70% example 2 ComparativeBPDA-TFMB DEEM N/A OXE02 N/A BTA N/A 39.0K 58% example 3 ComparativeBPDA-TFMB DEEM N/A OXE02 N/A TEGDMA N/A 36.0K 70% example 4 ComparativeBPDA-TFMB DEEM N/A OXE02 NPDEA TEGDMA N/A 35.4K 73% example 5Comparative BPDA-TFMB DEEM N/A OXE02 NPDEA TEGDMA APTES 32.3K 100% example 6 Comparative BPDA-TFMB DEEM N/A OXE02 N/A PTMA N/A 32.2K 100% example 7 Comparative BPDA-TFMB DEEM N/A OXE02 N/A TEGDMA N/A 31.6K100%  example 8 Comparative BPDA-TFMB DEEM N/A OXE02 N/A N/A N/A 31.6K100%  example 9 Comparative BPDA-TFMB DEEM N/A OXE02 N/A BTA N/A 31.6K98% example 10 Comparative BPDA-TFMB DEEM N/A OXE02 N/A PTMA N/A 31.6K100%  example 11 Comparative BPDA-TFMB DEEM N/A OXE02 N/A BTA N/A 28.8K98% example 12 Comparative BPDA-TFMB DEEM N/A OXE02 N/A N/A N/A 28.6K100%  example 13

The compositions of Examples 1-3 and Comparative Examples 1-13 are usedto prepare devices. Devices were formed with a 100 μm radius to test theformability of the parts. Polyimide solutions were roller coated ontostainless steel panels and then placed into a conveyor oven and dried.The resulting films were no longer tacky and were typically 20-24 μmthick. The films were then exposed to UV light through a photomask, postexposure baked, and developed in a NMP based organic developing solutionto form patterned features. The panels were then cured at 350° C. for 1hour under nitrogen. Cured film thickness was 10-12 μm. The panels werethen selectively etched by coating both sides of the panel withphotoresist, patterning with UV light, developing in aqueous base,etching in ferric chloride, and stripping the photoresist. The flat,etched parts were then mechanically formed using a fixed mandrel andsliding wipe punch with excess clearance. This is followed by asecondary cam punch to complete the form. The inner radius of the formis approximately 100 μm.

The formed parts were then inspected by optical microscope at 20× zoomin the region of the form. 40-100 parts for each example and comparativeexample were inspected. Parts where the polyimide edge had somesplitting at the form line or parts with complete breaks in thepolyimide layer were considered to have cracks. Parts with defectsoutside the formed area, but with no cracks in the formed area wereexcluded from the test.

As shown in Table 2, the devices of Examples 1-3, which include thelatent chain extender in its polymer formulation, had excellentformability. That is, the devices of Examples 1-2 did not have anycracks in its polyimide insulating layer. Example 3 demonstrated only10% cracks in its polyimide insulating layer formed from the polymerformulation of the present disclosure.

On the other hand, devices of Comparative Examples 2-13 showed poorformability and had at least 58% cracks in their polyimide insulatinglayer formed from polymer formulations without the latent chainextender. Comparative Example 1 demonstrated excellent formability andonly had 3% cracks. However, Comparative Example 1 has a very highweight average molecular weight of 59.8K. As discussed above, highweight average molecular weight polymers exhibit properties thatnegatively impact the initial manufacturing steps, and in particular thephotolithography processing steps. Comparative Example 1, therefore, isnot suitable for the initial processing steps prior to the cure step forforming the device.

Various modifications and additions can be made to the exemplaryembodiments discussed without departing from the scope of the presentinvention. For example, while the embodiments described above refer toparticular features, the scope of this invention also includesembodiments having different combinations of features and embodimentsthat do not include all of the described features. Accordingly, thescope of the present invention is intended to embrace all suchalternatives, modifications, and variations as fall within the scope ofthe claims, together with all equivalents thereof.

1. A photosensitive polymer formulation, comprising: a poly(amic acid) salt as a polyimide precursor; and a tertiary amine salt of a tetracarboxylic acid as a latent chain extender.
 2. The photosensitive polymer formulation of claim 1, wherein the poly(amic acid) salt includes (i) a base polymer of a dianhydride and a diamine, and (ii) a crosslinking agent.
 3. The photosensitive polymer formulation of claim 1, wherein the poly(amic acid) salt is a poly(amic acid) tertiary amine salt.
 4. The photosensitive polymer formulation of claim 2, wherein the dianhydride is BPDA.
 5. The photosensitive polymer formulation of claim 2, wherein the diamine is TFMB.
 6. The photosensitive polymer of claim 2, wherein the crosslinking agent is DEEM.
 7. The photosensitive polymer formulation of claim 2, wherein a weight average molecular weight of the base polymer is about 40,000 and less.
 8. The photosensitive polymer formulation of claim 2, wherein a weight average molecular weight of the base polymer is about 25,000-35,000.
 9. The photosensitive polymer formulation of claim 1, wherein the tertiary amine salt of a tetracarboxylic acid as the latent chain extender is prepared by reacting a dianhydride with water and a tertiary amine at room temperature.
 10. The photosensitive polymer formulation of claim 1, wherein the latent chain extender is prepared by reacting BPDA with water and DEEM at room temperature to form the tertiary amine salt of a tetracarboxylic acid.
 11. The photosensitive polymer formulation of claim 2, wherein a mol % of the diamine is higher than the mol % of the dianhydride.
 12. The photosensitive polymer formulation of claim 2, wherein a mol % of the dianhydride to a mol % of the diamine is in the range of 0.900 to 0.999:1.000.
 13. The photosensitive polymer formulation of claim 2, wherein a ratio of a mol % of the dianhydride and the latent chain extender to a mol % of the diamine is about 1.000:1.000.
 14. The photosensitive polymer formulation of claim 1, further comprising a photoinitiator.
 15. The photosensitive polymer formulation of claim 1, further comprising a sensitizer.
 16. The photosensitive polymer formulation of claim 1, further comprising a dissolution accelerator.
 17. The photosensitive polymer formulation of claim 1, further comprising an adhesion promoter.
 18. A method of forming a polyimide polymer, comprising: providing the photosensitive polymer formulation of claim 1; and curing the photosensitive polymer formulation, whereupon the latent chain extender is activated and reforms a dianhyride that reacts with terminal amine groups to form new imide bonds form the polyimide polymer.
 19. The method according to claim 18, wherein the poly(amic acid) salt includes (i) a base polymer of a dianhydride and a diamine, and (ii) a crosslinking agent.
 20. The method according to claim 19, further comprising controlling a mol % of the diamine to be higher than a mol % of the dianhydride to control a weight average molecular weight of the base polymer of the poly(amic acid) salt in the photosensitive polymer formulation.
 21. The method according to claim 19, further comprising controlling a ratio of a mol % of the dianhydride and the latent chain extender to a mol % of the diamine is about 1.000:1.000. 